The base excision repair pathway is responsible for repair of DNA lesions generated by ionizing radiation, oxidative damage, and alkylation damage. It has not been investigated as thoroughly as has the nucleotide excision repair pathway. DNA alkylating agents used in chemotherapy generate cytotoxic DNA lesions which are responsible for the tumor killing effect of these agents. The 3-methyladenine-DNA glycosylases from bacterial, rodent, and mammalian sources catalyze the first repair step, base excision, of many alkylated lesions. To identify the range of alkylated substrates repaired by these three enzymes, DNA will be modifies by a variety of alkylating agents including chemotherapeutic agents such as nitrogen mustards, chloroethyl-nitrosoureas, and cyclophosphamide. Rates of cleavage, in vitro repair, at each base position will be determined for bacterial, rat, and human 3meAdenine glycosylases. In vivo repair rates for dimethyl sulfate and bischloroethylnitrosourea will be mapped at the nucleotide level of resolution along the PGK1 promoter and p53 exons in normal human male fibroblasts using the ligation-mediated Polymerase Chain Reaction (LMPCR). These data will provide information on sequence context and chromatin dependence for base excision repair rates. A PCR-based primer extension assay to replace the Bohr-Hanawalt assay for repair rates, will be developed to produce a population screening assay for activity of all major repair pathways. This assay will be used to test the hypothesis "Many brain tumors resistant to a combination of bischloroethylnitrosourea and 06benzylguanine are resistant because they overexpress 3meadenine DNA glycosylase activity."